advice?

Well, first off, you should know that he is at the prime age to have separation anxiety and it isn't because you are his only parent or anything like that, it is normal for all kids. One of the best things that you can do to help him with his anxiety to to work together with one of your friends that is also an adult (doesn't matter if they have children or not) to make sure that he is comfortable with at least one other adult.

Yeah - even though I have kids of my own - I don't know off the top of my head if this is normal for that age... I'd definitely look up some resources online to find out. Either way though, how you respond to his 'flipping out' will - in my opinion - tell him how justified his screaming was. Ya know - like if a kids hands are dirty and someone freaks out and grabs the child and is all panicky while they quickly apply gallons of hand-sanitizer to the child's hands... having that lesson reinforced on semi-regular basis will teach the child to freak out themselves.... Anyway, I'd just apply basic care and attention without being overly concerned. Give him a hug, tell him you're sorry that he was scared by your leaving for a moment, let him know his feeling are important... but, ya know - don't freak out into a spasm of uncontrollable coddling until he quiets down... otherwise I think this is a way that mothers teach the child that it really is scary to be outside the immediate presence of their mother. Anyway... some thoughts for you.

they say if you play hiding seek with a baby that young it suppose to help with this.

I think it's normal for 9 months old boy to screams around. I don't want to continue my writings coz i'll end-up lecturing....